1. Field of the Invention
The present invention relates generally to demodulation of communication signals and more particularly to an interpolating bandpass filter for packet-data receiver synchronization.
2. Description of the Background Art
In network communications a carrier signal, a signal that oscillates at a fixed frequency, is used to establish the base frequency at which data signals are transmitted. These data signals modulate the carrier frequency prior to transmission.
Normally, it is desirable to demodulate data signals from their carrier communication signal using a digital method. It is further desirable to sample the communication signal at a rate greater than two times the maximum frequency of the modulated signal. This process is known as passband sampling.
Typically, demodulating a sampled data stream requires a xe2x80x9cmatched filterxe2x80x9d for optimum performance. This xe2x80x9cmatched filterxe2x80x9d is usually implemented as a bandpass filter with the signal being sampled in the passband. A bandpass filter is an electronic device that prohibits all but the specific range of frequency of the desired modulation to pass through, thus rejecting interference and noise that may be present in the sampled data stream. Often, this xe2x80x9cmatched filterxe2x80x9d is implemented as a finite-impulse response (FIR) digital filter.
It is common for a demodulator to require a sample rate to be interpolated to a different rate for later application to an equalizer structure. This sample rate change is necessary when the original sampling rate is not the same frequency as the sampled rate in the receiver structure. Typically, the receiver structure requires synchronization with the far-end transmitter. This synchronization is usually accomplished with a phase-locked loop on the clock to the analog-to-digital converter or some interpolating FIR structure on the received sample stream. Conventionally, the xe2x80x9cmatched filterxe2x80x9d and the interpolation functions are performed by separate and distinct elements in a receiver. Some transmission standards, particularly those that involve more than two transceiver units, mandate that data are transmitted in packets. An example of such a transmission standard is described in the Home Phoneline Network Alliance (referred to herein as xe2x80x9cHomePNAxe2x80x9d or xe2x80x9cHPNAxe2x80x9d) 2.0 standard. The HPNA standard requires a receiver to synchronize its local clock to that of the remote transmitter clock for the duration of the transmitted packet. Details regarding the HPNA 2.0 standard are disclosed in xe2x80x9cInterface Specification for HomePNA 2.02.7 10M8 Technologyxe2x80x9d by Home Phoneline Networking Alliance, herein incorporated by reference.
In general, the present invention provides an interpolating bandpass filter capable of synchronizing packet data received by the receiver device. The system of the invention includes a single multi-tapped delay line that delays the passband-sampled data from the analog-to-digital converter. A convolving structure in conjunction with the multi-tapped delay line implements a finite impulse response (FIR) filter, which has multiple coefficient sets. The coefficient sets are selected to implement an L-times oversampling bandpass filter.
A separate-accumulator accumulates a fractional phase difference that represents the frequency offset between the remote transmitter""s frequency and the receiver""s sampling frequency. The integer portion of the phase difference is used to determine the number of samples to delay the input of the FIR filter, while the fractional portion is used to choose two filter sets closest to the desired delay and to interpolate between these two filter sets.
In the preferred embodiment, a coefficient generator stores 32 coefficients divided into even and odd coefficients. The fractional portion of the phase difference is split into two components. One component determines a subset of coefficients from the even coefficients and a subset of coefficients from the odd coefficients, and the other component is used to interpolate between these two subsets to provide coefficients that are convolved with input samples.
Other advantages, features, and embodiments of the present invention will be apparent from the drawings and detailed description as set forth below.